Like the idea - use a board to save the walls and maybe not Velcro since it'll get gross and isn't easily cleaned. Magnets or even small hole in cup to hang on a hook? Cars, Trucks and Teething Rings: Kids Toothbrush/Bathroom Fix
Use sticky Velcro and travel toothbrush holders to store them inside the bathroom cabinet. When it's time to use them, just pull them off pop the top on and go. The rough side of the Velcro will be waiting to stick them to when you get back.
I found the perfect use for our extra cord catchers ($3.99 for 6 at Walmart) and finally the perfect storage solution for kids toothbrushes that are too wide for normal toothbrush storage. Woot! #MommyWin